1. Field of the Invention
The present invention relates to a transmitter, more particularly to a transmitter employing an FDD (Frequency Division Duplex) system.
2. Description of the Prior Art
FIG. 1 is a block diagram showing a conventional transmitter disclosed in Japanese Patent Laid Open No. 174018/89.
The transmitter shown in FIG. 1 consists of a radio transmitter-receiver and a transmitter-receiver detouring wave canceling circuit. The radio transmitter-receiver comprises transmitting signal input terminal 21, transmitting IF amplifier 22, transmitting frequency mixer 23, transmitting filter 24, transmitting power amplifier 25, local oscillator 26, distributer 27, transmitter-receiver commonly-used device 28, antenna connecting terminal 29, receiving filter 30, receiving high-frequency amplifier 31, receiving frequency mixer 32, receiving IF filter 33, receiving IF band amplifier 34, and receiving signal output terminal 35. The transmitter-receiver detouring wave cancel circuit comprises directional coupling device 36, adjusting device 37, directional coupling device 38, transmitting filter 39, detector 40, and control section 41.
Transmitting waves are partially branched by directional connection device 36 disposed in a transmitting side, and the branched waves are output to directional coupling device 38 disposed in a receiving side via adjusting device 37. Hereupon, adjusting device 37 adjusts the amplitude and the phase of the transmitting waves output from directional coupling device 36 so that the amplitude of the transmitting waves is the same as that of the transmitter-receiver detouring waves and the phase of the transmitting waves shifts from that of the transmitter-receiver detouring waves by 180 degrees. The transmitter-receiver detouring waves are output to directional coupling device 38 after passing through transmitter-receiver commonly-used device 28. Directional coupling device 38 disposed in the receiving side synthesizes the waves output from adjusting device 37 with the transmitter-receiver detouring waves which detours around adjusting device 37 by passing through transmitter-receiver commonly-used device 28. As a result, the transmitter-receiver detouring waves are canceled.
The foregoing conventional transmitter has heretofore employed the technique in which the amplitude and the phase of the signal obtained by separating the transmitting output are adjusted and then the signal is synthesized with the receiving input. However, this technique requires directional coupling devices both for the transmitting and receiving sides so that transmitting loss is brought about and a power amplification efficiency is made worse in the coupling device of the transmitting side. On the other hand, in the coupling device of the receiving side, a receiving signal is attenuated, and a receiving sensitivity is deteriorated. Furthermore, it is required to detect transmitting noises included in a receiving IF band and to adjust the amplitude and phase of the branched transmitting signal with a high precision. Hence, there is a problem that the circuit constitution is complicated.